1. Field of the Invention
This invention relates generally to the apparatus for producing the timing signals in a data processing system and, more particularly, to data processing systems that have a system clock with a variable frequency and require a constant frequency clock source to control the operation of interval timers and related apparatus.
2. Description of the Related Art
A data processing system generally has a plurality of components that must cooperate in the processing of digital signal groups. An example of such a data processing system is shown in FIG. 1. In this data processing system, at least one central processing unit 10 (or 11), at least one input/output unit 13 (or 14) and a memory unit 15 are coupled by means of a system bus 19. A console unit 12 can be coupled to the central processing unit(s). The central processing unit manipulates groups of logic signals, representing data information, according to control signals in the form of groups of logic signals representing instructions. These instructions are typically components of software or firmware programs. The memory unit provides the principal mechanism for storage of data signal groups and program signal groups to be manipulated by the central processing unit(s). The console unit can provide for the initialization of the data processing system and, during the operation of the data processing system, can function as a terminal. The console unit is frequently used for the control of diagnostic procedures for the data processing system. The input/output unit 13 provides the interface for exchange of signal groups between the data processing system and mass memory storage units, terminal devices, communication devices, and other peripheral devices requiring interaction with the central processing unit(s).
The components of the data processing system must be coordinated in order to provide consistent performance. This coordination is typically performed by a timing or clocking mechanism. For asynchronous data processing systems, individual components of the data processing system can have associated clocking mechanisms. For asychronous data processing systems, interface units are required to insure the integrity of the data signal groups during the exchange of signal groups between the individual components. For synchronous data processing systems, the entire data processing system can have a single system clock by which means the flow of data signal groups throughout the entire data processing system can be coordinated.
It can be desirable to provide a system clock that can provide signals having a variable frequency. The variable frequency signals can permit the rate of processing of the logic signal groups to be increased, as the frequency of the system clock is increased, or to be decreased, as the frequency of the system clock is decreased. This functionality can be particularly advantageous in the detection of system malfunctions because the system can be operated at a rate that can permit the detailed analysis of particular processing functions. This functionality can also be used to increase the power of the entire data processing system when a component or unit, limiting the frequency of the system, can be replaced with an improved component or unit that can operate at an increased frequency.
In addition to the system clock, data processing systems typically include an interval timer. The function of the interval timer is to provide timing signals that are fixed in frequency and can therefore be used to provide real time measurements for such purposes as maintaining a calendar, measuring time for project execution, supplying billing information, etc. The interval timer must be driven by a constant frequency clock source even though the system clock can have a variable frequency. Furthermore, the timing signals used by the interval timer must be maintained in phase synchronization with the signals of the system clock. It has been known in the related art to provide two reference frequency oscillators. The output signals of the first oscillator can be processed in a predetermined manner to provide the constant frequency signal source, while the output signals from the second oscillator can be processed in a controllable manner to provide the variable frequency signal source. This technique suffers from the difficulty in synchronizing the two sets of signals and insuring that the two sets of signals are in phase.
A need has therefore been felt for a clocking apparatus that can provide signals used for system clocking purposes that have a controllable frequency, while simultaneously providing constant frequency signals for the system interval timer and related apparatus that are in phase with the system clock signals.